In order to develop a high hardness cutting tool material, various multilayer film systems based on TiN have been proposed since the late 1980's.
For example, when a multi-layered film is formed by alternately and repeatedly laminating TiN or VN with a thickness of several nanometers, despite the differences in the lattice constants of single layers, a matching interface is formed between the layers to form a so-called superlattice having one lattice constant, and thus, a high hardness of at least two times the general hardness of each single film may be achieved. A variety of attempts have been made to apply this phenomenon to a thin film for a cutting tool.
Reinforcing mechanisms used for such superlattice coating include the Koehler's model, the Hall-Petch relationship, the coherency strain model, and the like, and these reinforcing mechanisms increase hardness by controlling the differences in lattice constants and elastic moduli of materials A and B and a lamination period when the materials A and B are alternately deposited.
Recently, for example, as disclosed in Patent document 1 (Korean Patent Publication No. 2013-0123238), hard coatings for a cutting tool provided with various nano-multilayer structures, in which nitrides with various compositions such as AlTiN, TiAlN, AlTiMeN (where, Me is a metal element) are alternately laminated to achieve a remarkably improved physical property compared to a single film, have been proposed.
However, nitride thin films such as AlTiN, TiAlN, or AlTiMeN have limitations in that a phase decomposition into AlN, TiN, or MeN occurs due to a high temperature and a high pressure during a cutting process, and since the difference in thermal expansion coefficients of phase-decomposed AlN, TiN, or the like is excessively large, a thermal crack easily occurs in an initial stage during a cutting process such as medium-to-low speed interrupted milling processing and thereby reduces the tool life.